1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer, and a facsimile, in which an image is formed by utilizing an electrophotographic type, an inkjet type, and the like. Particularly the invention relates to the image forming apparatus which can detect density of the image on the recording medium to perform image density adjustment.
2. Description of the Related Art
As is well known, in the electrophotographic type image forming apparatus, a visible image borne on a photosensitive drum or an intermediate transfer member is transferred to a recording medium (sheet) such as plain paper to obtain a recording image. Therefore, the recording medium to which the visible image is transferred from the photosensitive drum or the intermediate transfer member is conveyed to a fixing unit to subject to the fixation of the visible image. Then, the recording medium is discharged.
Recently, the needs for a high-quality image and a high-stabilization image are being increased in the image forming apparatus. In order to always keep the density of the image formed by the image forming apparatus an appropriate state, there are proposed many technologies which perform image density control. For example, in the conventional technology, a reference image having predetermined density is formed, the density of the reference image is measured, the measured value of the reference image is compared to a predetermined target density value to produce a conversion table, and the density control of the image is performed by converting density characteristics of image data with the conversion table.
Usually the reference image density necessary for the density control is measured before a toner image is transferred to the record medium, or namely, the reference image is formed on the photosensitive drum or the intermediate transfer member to measure the density of the reference image. The toner image is the visible image formed as the reference image. However, in the density measuring method, due to the fluctuation in toner amount to the recording medium and the fluctuation in degree of fixation of the toner, there is generated difference in density between the image on the actually obtained recording medium and the measured image on the photosensitive drum or the intermediate transfer member. Therefore, density control cannot be performed with high accuracy.
In order to solve the above problem, there is proposed the image forming apparatus, in which the reference image is previously formed in the recording medium, an image reading unit reads the reference image, and the image density is controlled based on the read result of the reference image. Further, Japanese Patent Application Laid-Open No. 2000-132013 discloses the image forming apparatus, in which an optical density sensor is provided in a conveyance path for the recording medium after the transfer or fixation, the density sensor detects the reference image previously formed on the recording medium during conveyance, and image density adjustment is performed based on the detection information.
In the inkjet type image forming apparatus, the image density is also changed by variation with time and environmental difference of the amount of ink ejection, individual differences of ink cartridges, and the like. Therefore, there is proposed the image forming apparatus, in which the optical density sensor is arranged near a discharge unit of the image forming apparatus, the density sensor detects the density of the image on the recording medium during the conveyance, and image density adjustment is performed based on the detection information.
However, in many reference images formed by the conventional image forming apparatus, plural reference patches are arranged at regular intervals in the recording medium conveyance direction, and the reference patches arranged at regular intervals are detected at constant timing by the density sensor. Therefore, the following problem is generated.
When the test pattern image in which the plural reference patches is detected at regular intervals, because a recording medium front-end enters the bent conveyance path, a roller pair and a sensor flag in the conveyance path, and the like while the density sensor measures the density, the recording medium fluctuates (vibrates) in a vertical direction (direction in which the recording medium is brought close to and separated from the density sensor) with respect to the density sensor. Therefore, sometimes the density measurement is difficult to perform with high accuracy. This is because usually an output value of the optical density sensor is largely changed when a measurement object fluctuates vertically.
In order to suppress the vertical fluctuation of the recording medium, it is thought that an allowance for the vertical fluctuation of the recording medium is widened in the density sensor by using an optical system combining a light source, a lens, and a light reception element for the density sensor. Therefore, large effect can be obtained by increasing a lens diameter or using plural lenses.
However, the above countermeasure leads to upsizing of the density sensor, the increase in the cost of components, the increase in the number of components, and the increase in the man-hour, which results in the problems that the density sensor size is increased and production cost for the density sensor is increased. These problems also become a large obstacle to downsizing and cost reduction in the image forming apparatus as a whole. Further, in the countermeasure, only the allowance for the vertical fluctuation of the recording medium is widened in the density sensor, so that the image density adjustment is difficult to perform with high accuracy in the system in which the recording medium largely fluctuated in the vertical direction.